1. Field of the Invention
The present invention relates generally to methods and compositions for identifying immunosuppressive drugs. More particularly, the present invention relates to the identification of drugs which can inhibit T cell-mediated immune response by blocking stimulation of the CD28 signal transduction pathway.
The cellular immune response involves a very complex set of interactions between antigens, T cells, B cells, macrophages, and numerous factors, such as cytokines, which are released by the cells during the course of the interactions. The present invention is concerned particularly with T cell activation which results from interaction with particular antigen presenting cells, such as B cells and macrophages. While specificity of the T cell response is determined by antigen-specific binding to the T cell antigen receptor (TCR), binding to at least one secondary receptor is also necessary for activation. One such secondary receptor is CD28 which, upon stimulation, induces the activity of nuclear proteins which can increase the production of interleukin-2 and possibly other cytokines by binding to an enhancer region associated with the cytokine gens.
Heretofore, most efforts in identifying immunosuppressive drugs which meditate the cellular immune response at the level of intracellular interaction have focused on inhibiting binding to the TCR. While such efforts have met with some success, it would be desirable to provide compositions and methods useful for identifying immunosuppressive drugs which can interfere with binding to secondary T cell receptors, particularly CD28. Such methods should be convenient, inexpensive, rapid, and should permit screening of a large number of candidate drugs under controlled conditions.
2. Description of the Background Art
Weiss et al. (1986) J. Immunol. 137:819-825, describes the ability to activate purified T cells and Jurkat cells by exposure to anti-CD28 antibodies and certain T cell receptor stimulants. Durand et al. (1988) Mol. Cell. Biol. 8:1715-1724, describes the construction of pIL-2-Luc. Fraser et al. (1991) Science 251:313-316, describes the effects of CD28 stimulation on IL-2 enhancer activity in Jurkat cells transiently transfected with pIL-2-Luc. Exposure of the transfected cells to anti-CD28 and TCR stimuli resulted in enhanced luciferase activity. A ligand for stimulating the CD28 receptor present on activated B cells (B7/BB1) is described in Lindsley et al. (1990) Proc. Natl. Acad. Sci. USA 87:5031-5035. The importance of the CD28 costimulatory pathway is described in Fraser et al. (1992) J. Exp. Med. 175:1131-1134.
The full disclosure of each of these references is incorporated herein by reference.